Ondura installation paper thingy says to use a circular saw with a
carbide tooth blade if you wants to cut off some of the lenght.(the
stuff is made of apshalt (now, just watch somebody pipe up and told me
how to spell that!) with some kind of fibers in it)

I tried that with my old Skilsaw (SKIL, Trademark Registered, TM) with a
carbide blade I put on my saw a year ago.

I tell y'all what, don't do it! Instead, use an old jagged edge (factory
made jagged edge/sawtooth edge/whatever you wants to call it) kitchen
knife, it works like a Charm! Make a couple of marks with a pencil, lay
another piece of Ondura on the other Ondura and draw your pencil line
where you wants to cut, along the corrugated ridges and also down in the
valleys.Git your jagged edge kitchen knife and start cutting.

Be sure to clean off the knife real good so your wife or galfriends wont
know what you used it for.
cuhulin, done learned the Best way to cut Ondura roofin

Down innnnn the Valleyyyyyy,,,,,, Valley so lowwwwww,,,,,,, hang your
head lowwwww, dear, hang your head lowwwwww,,,,,,,,, Roses lovvvve
Sunnnnnshinne,,,, Violets love dewwwwww,,,,,,